Go to Admin > Config > Core Set Up > School to check the lowest and highest year groups listed here. This error would indicate the student is outside of these year groups. If the highest or lowest year group is incorrect in the school details here please edit and update to remove the error.
Please note in this area:
E1 = Early First Year = -4
E2 = Early Second Year = -3
N1 = Nursery First Year = -2
N2 = Nursery Second Year = = -1